Compositions and methods for separating a sample of whole blood into a substantially non-activated platelet rich plasma fraction (PRP fraction) and at least one other fraction are provided. A sample of whole blood can be centrifuged in a collection tube comprising a separator substance formulated to settle between the PRP fraction and the at least one other fraction. Preferably, centrifugation is completed without substantial activation of platelets. Optionally, the separator substance could be hardened to form a solid barrier that allows removal of all or substantially all of the platelets in the PRP fraction without remixing of the PRP fraction with the at least one other fraction.
